Block allocation map
GPFS has two different methods of allocating space in a file system. The -j parameter specifies the block allocation map type to use when creating a file system. The block allocation map type cannot be changed once the file system is created.
When allocating blocks for a given file, GPFS first uses a round-robin algorithm to spread the data across all of the disks in the file system. After a disk is selected, the location of the data block on the disk is determined by the block allocation map type.
- cluster
- GPFS attempts to allocate
blocks in clusters. Blocks that belong to a given file are kept next
to each other within each cluster.
This allocation method provides better disk performance for some disk subsystems in relatively small installations. The benefits of clustered block allocation diminish when the number of nodes in the cluster or the number of disks in a file system increases, or when the file system free space becomes fragmented. The cluster allocation method is the default for GPFS clusters with eight or fewer nodes and for file systems with eight or fewer disks.
- scatter
- GPFS chooses the location
of the blocks randomly.
This allocation method provides more consistent file system performance by averaging out performance variations due to block location (for many disk subsystems, the location of the data relative to the disk edge has a substantial effect on performance). This allocation method is appropriate in most cases and is the default for GPFS clusters with more than eight nodes or file systems with more than eight disks.
This parameter for a given file system is specified at file system creation by using the -j option on the mmcrfs command, or allowing it to default. This value cannot be changed after the file system has been created.